A new model to derive the transport parameters in CZT detectors with a liner decreasing internal electric field

Charge collection efficiency and material transport properties in CdTe and CZT planar devices for X and gamma ray detection are commonly determined by means of the Hecht equation, under the limiting approximation of a uniform internal electric field. As an alternative, the Manys theory is helpful when surface recombination velocity of carriers has to be taken into account. Experiments carried out on planar detectors have shown a non constant, linearly decreasing profiles of the internal electric field and, in several cases, the electric field does not decrease down to zero but to a fixed value different and afterwards it result to be constant. A similar non-uniform electric field could be also expected when a high radiation flux hits the surface near the detector contacts. This fact could explains as photoconductivity measurements on these kinds of devices are often not so easy described by means of the Hecht theory. Last of all, the use of the Hecht or Manys equations to fit photoconductivity curves could lead to wrong conclusionsin the determination of ?? product. Starting from the Ramo-Shockley theorem, the authors calculate a new relation between charge collection efficiency and applied bias in the case of an electric field decreasing linearly along the detector thickness, being satisfied all the other Hecht hypotheses. The new functional dependence of charge collection efficiency on applied bias contains the slope of the electric field as a parameter and provides for the Hecht model as the special solution when the electric field is uniform. Moreover this model allows to fit with a very good accuracy experimental results on several our CZT detectors. The authors believe that this shape of field should be widespread in presence of diffused spatial charge and think that this model could be an important instrument for interpreting the data arising from pulsed photocurrent measures not only for CdTe and CZT detectors but also for other materials.